Chapter 3: The Molecules of Cells – Structure and Function of Biological Macromolecules

Introduction to Organic Compounds and Their Polymers

Biological diversity and function are rooted in the molecular diversity of cells. The ability of organisms to perform complex tasks, such as digesting dairy or building spider silk, depends on the presence and structure of specific biological molecules.

• Lactose intolerance is caused by the absence of the enzyme lactase, illustrating the importance of enzymes in metabolism.

• Spider silk's strength and resilience are determined by the structure of silk proteins, which are encoded by DNA.

3.1 Life’s Molecular Diversity Is Based on the Properties of Carbon

The Central Role of Carbon

Carbon is the foundational element of organic molecules due to its unique bonding properties.

• Organic compounds are molecules containing carbon and are essential to life.

• Carbon chains form the backbone of most organic molecules, allowing for a variety of structures.

• Isomers are compounds with the same molecular formula but different structures, leading to different properties.

• Hydrocarbons consist only of carbon and hydrogen atoms.

A carbon atom forms four covalent bonds, enabling it to create chains or rings. The four single bonds of carbon point to the corners of a tetrahedron.

• Carbon skeletons can vary in length, branching, presence of double bonds, and ring formation.

Examples of Carbon Skeletons

Type	Example	Description
Length	Ethane, Propane	Skeletons vary in length
Branching	Butane, Isobutane	Skeletons may be unbranched or branched
Double Bonds	1-Butene, 2-Butene	Skeletons may have double bonds in different locations
Rings	Cyclohexane, Benzene	Skeletons may be arranged in rings

3.2 Functional Groups Help Determine the Properties of Organic Compounds

Key Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

• Hydroxyl group (-OH): Characteristic of alcohols; makes molecules hydrophilic.

• Carboxyl group (-COOH): Acts as an acid; found in amino acids and fatty acids.

• Amino group (-NH2): Acts as a base; found in amino acids.

• Carbonyl group (C=O): Found in sugars (as aldehydes or ketones).

• Methyl group (-CH3): Nonpolar; affects gene expression.

• Phosphate group (-PO4): Found in nucleic acids and ATP; involved in energy transfer.

The properties of organic molecules depend on the size and shape of their carbon backbone and the functional groups attached.

Table: Some Common Functional Groups

Functional Group	General Formula	Name of Compound	Example	Where Else Found
Hydroxyl	-OH	Alcohols	Ethanol	Sugars
Carbonyl	C=O	Aldehydes/Ketones	Acetone	Sugars
Carboxyl	-COOH	Carboxylic acids	Acetic acid	Amino acids, fatty acids
Amino	-NH2	Amines	Glycine	Amino acids
Methyl	-CH3	Methylated compounds	Methylamine	DNA, proteins
Phosphate	-PO4	Organic phosphates	ATP	Nucleic acids

Example: Sex Hormones

• Testosterone and estradiol differ only in the functional groups attached to their carbon skeletons, yet have vastly different effects in the body.

3.3 Cells Make Large Molecules from a Limited Set of Small Molecules

Macromolecules and Polymers

Most biological molecules are macromolecules, which are large polymers made from smaller units called monomers.

• Polymer: Long chain of monomers.

• Monomer: Small molecular unit that is the building block of a polymer.

Cells use a small set of monomers to build a wide variety of polymers.

Polymerization and Depolymerization

• Dehydration reaction (synthesis): Links monomers together by removing a molecule of water to form a new bond.

• Hydrolysis: Breaks polymers into monomers by adding a molecule of water, breaking a bond.

• These reactions are catalyzed by enzymes.

Example: When you eat cheese, proteins are broken down into amino acids by hydrolysis, and your cells can then use dehydration synthesis to build new proteins from these amino acids.

Carbohydrates

Overview

Carbohydrates are a class of molecules that include sugars and polymers of sugars. They serve as fuel and building material for cells.

• Range from small sugars (monosaccharides) to large polysaccharides (e.g., starch, cellulose).

3.4 Monosaccharides: The Simplest Carbohydrates

• Monosaccharides: Single-unit sugars (e.g., glucose, fructose).

• General formula: $(CH_2O)_n$

• Contain hydroxyl and carbonyl groups.

• Serve as the main fuel for cellular work.

• Glucose and fructose are isomers (same formula, different structure).

• Monosaccharides often form ring structures in aqueous solutions.

3.5 Disaccharides: Double Sugars

• Formed by joining two monosaccharides via a dehydration reaction.

• Examples: Sucrose (glucose + fructose), Maltose (glucose + glucose).

3.6 Connection: Are We Eating Too Much Sugar?

• The FDA recommends that only 10% of daily calories come from added sugar.

• High sugar intake is correlated with adverse health effects.

• "Empty calories" refers to foods high in sugar but low in essential nutrients.

3.7 Polysaccharides: Complex Carbohydrates

• Polysaccharides are long chains of monosaccharide units.

• Starch: Storage polysaccharide in plants.

• Glycogen: Storage polysaccharide in animals.

• Cellulose: Structural polysaccharide in plant cell walls.

• Chitin: Structural polysaccharide in arthropod exoskeletons and fungal cell walls.

Comparison of Starch and Cellulose

Property	Starch	Cellulose
Function	Energy storage (plants)	Structural (plant cell walls)
Monomer	Glucose (α-linkage)	Glucose (β-linkage)
Digestibility	Digestible by humans	Indigestible by humans

Summary Table: Carbohydrates

Type	Examples	Function
Monosaccharides	Glucose, Fructose, Galactose	Fuel for cells
Disaccharides	Sucrose, Maltose, Lactose	Transport form of sugar
Polysaccharides	Starch, Glycogen, Cellulose, Chitin	Storage and structure

Additional info: This summary covers the first half of Chapter 3, focusing on the chemical basis of life, the diversity of organic molecules, and the structure and function of carbohydrates. For a complete understanding, students should also study lipids, proteins, and nucleic acids, which are covered in the latter part of the chapter.